For many applications in which alloys are employed, resistance to deformation is necessary, in addition to resistance to oxidation, for service under conditions of cyclic and continuous thermal stress at temperatures far above 900.degree. C. Electric resistance heating elements are examples of such applications.
Iron-chromium-aluminum alloys are equal or superior to austenitic resistance alloys, owing to their high specific electric resistance (values of up to 1.6 Ohm.mm 2.sup.2 m.sup.-1 are known) and to their resistance to scaling. However, the austenitic nickel-chromium alloys exhibit clear advantages as to creep behavior when heated by electric current at temperatures above 1000.degree. C. Therefore, the improvement of creep properties, i.e. of creep elongation under thermal stress, of iron-chromium-aluminum alloy steels are of technical and economic advantage. These more favorably-priced steels could usefully be employed in an extended temperature range.
Yttrium additions between 0.01 and 3% are known to improve the creep properties of iron-chromium-aluminum steels (all percentages are mass percentages).
German Offenlegungsschrift No. 29 16 959 describes the improvement of hot gas corrosion behavior by yttrium and silicon additions. The carrier of these properties is preferably the .alpha.-Al.sub.2 O.sub.3 produced at temperatures of more than 1000.degree. C. at the surfaces of heating elements with resistance heating and support foils in exhaust gas purification devices (catalysts) of motor vehicles. Apart from the financial aspect raised by the high production costs of the yttrium prealloy, this element--when used in iron-chromium-aluminum alloys--presents the disadvantage of reducing the maximum application temperature to about 1250.degree. C. This is due to the eutectic compositions being formed in the binary system yttrium/iron, e.g. between YFe.sub.4 and YFe.sub.5. The relevant details were described by R. F. Domagala, J. J. Rausch and D. W. Levinson in Trans. ASM 53 (1961), p. 137-155, and by R. P. Elliott in "Constitution of Binary Alloys (lst supplement)" McGraw-Hill Book Company, New York, (1965), page 442, FIG. 231 Fe-Y.
The fundamentals for resistance to scaling were described by H. Pfeiffer and H. Thomas in "Zunderfeste Legierungen", Springer Verlag 1963, 2d edition, Berlin/Gottingen/Heidelberg, pages 248 and 249. The alumina (Al.sub.2 O.sub.3) mainly performs a protective function against oxidation. Conditions above 1000.degree. C. are most important for the working life. On evaluating the working life, i.e. the duration of the cyclic or continuous thermal stress, special importance has to be attached to the adhesiveness between the metallic sectional area and its Al.sub.2 O.sub.3 coating. The alumina layer offers optimum protection, if its density is high and if the oxide does not spall. Due to the different thermal expansion coefficients of the metallic matrix and of the oxide, however, spalling of the oxide layer is likely to occur to a greater or lesser extent. J. Peters and H. J. Grabbke ("Werkstoffe und Korrosion" 35 (1984) p. 385 to 394) have examined the influence of oxygen-affinitive elements on ferrous alloys, with the result that titanium containing steels alloyed with chromium and aluminum form a good protective coating. This favorable behavior is the consequence of the formation of a fine-grained alumina coating on a titanium oxicarbide layer between Al.sub.2 O.sub.3 and ferrite.
Furthermore, U.S. Pat. No. 4,414,023 describes a heat-resistant ferritic and non-corrosive steel alloy which is a steel which contains 8 to 25% chromium, 3 to 8% aluminum and an addition of rare earth metals, viz. cerium, lanthanum, neodymium, praseodymium, and other elements of this group within the range of composition between 0.002% and max. 0.06% and which forms an adhesive composition between the oxide and the ferritic matrix. It was especially emphasized that
(a) titanium additions have no negative influence,
(b) zirconium has no or merely a slightly positive influence on the adhesiveness in case of concentrations of up to 0.008%,
(c) only one element should be used, since in the case of complex alloys, the element with the most negative influence determines the protective function against oxidation.
U.S. Pat. No. 3,992,161 discloses a steel which contains 10 to 40% Cr, 1-10% Al, up to 10% Ni, up to 20% Co, up to 5% Ti, up to 2% of each of rare metals Y, Zr, Nb, Hf, Ta, Si, V, up to 6% of W and Mo respectively, up to 0.4% C, up to 0.4% Mn and 0.1 to 10% by volume of a dispersoid of the group metal oxide, metal carbide, metal nitride, metal boride, remainder iron. This steel is especially designed for the production of resistance heating elements, blades and combustion chambers of gas turbines. The essential component for achieving the desired strength properties of this steel is the dispersoid, the total contents of which amounts to--according to the embodiments--about 1% of the weight of the steel. Dispersoids increase the strength, albeit at the expense of ductility. However, they deteriorate the workability considerably. Therefore, an increased volume of finishing work is required, since surface defects will occur to a greater extent during the processing due to the lower purity degree of the steel. These surface defects have to be eliminated by grinding, so that high costs are incurred consequently. A special disadvantage of U.S. Pat. No. 3,992,161 is the costly powder-metallurgical process which is only worth considering in order to obtain the necessary fine distribution of the dispersoids within the range of 50 to 5000 .ANG.. A sufficiently fine distribution of the dispersoids cannot be achieved in metallurgical melting processes. Moreover, the known steel has poor welding properties due to the contents of dispersoids and a low creep resistance under thermal stress.